Ok, I'll bite, please dont bite my head off. I know there are people much more edumacated in this. maybe they will step up and teach us both.

the higher the tension in the batten the larger the static draw of the sail will be. depending on the cut of the sail. sorta like outhaul. but not easily adjustable.

the bigger the draw. the more power the sail will have. the more draw you have the slower tha sail will be. power and speed are different.

a bacic example of this would be a heavy mono hull would have more draw/power to move the wieght and since the inherit speed is lower. while faster boat (catamaran) would have a flatter sail with less draw.

sailing up-wind basicly you want a flat sail. with low batten tension and the out haul tight

sailing down wind you want a sail that is more full higher batten tension with the out haul loose.

the trick is setting the batten to how the sail is cut,and the conditions you are sailing in and your prefernce.

i sail mostly in the ocean and prefer a higher batten tension than most. i prefered the power for the seas and chop. but you give back top end speed.

I used the guide from murray's when i was first experimenting with this. If you have the time play with batten tensions in the same conditions till you find a grove that you like. cause everything is related sailcut,batten tension,outhaul,windspeed and conditions.